1. Field of the Invention
This invention relates to a process for effecting isomerization of an aromatic C.sub.8 mixture comprising ethylbenzene and xylene in the presence of a specified crystalline aluminosilicate zeolite catalyst characterized by a crystal size of at least about 1 micron.
2. Description of the Prior Art
Xylenes are valuable industrial chemicals. They are derived primarily from such aromatic naphthas as petroleum reformates and pyrolysis gasolines. The former result from processing petroleum naphthas over a catalyst such as platinum on alumina at temperatures which favor dehydrogenation of naphthenes. Pyrolysis gasolines are liquid products resulting from steam cracking of hydrocarbons to manufacture ethylene, propylene, etc.
Generally and regardless of aromatic naphtha source, it has been the practice to subject the liquid hydrocarbon to extraction with a solvent highly selective for aromatics to obtain an aromatic mixture of the benzene and alkylated benzenes present in the aromatic naphtha. The resulting aromatic extract may then be distilled to separate benzene, toluene and C.sub.8 aromatics from higher boiling compounds in the extract. Benzene and toluene are recovered in high purity but the C.sub.8 fraction, containing valuable para xylene, is a mixture of the three xylene isomers with ethylbenzene. Techniques are known for separating p-xylene by fractional crystallization with isomerization of the other two isomers in a loop to the p-xylene separation. That operation is hampered by the presence of ethylbenzene. However, a widely used xylene isomerization technique, "Octafining" can be applied. Octafining by passing the C.sub.8 aromatics lean in p-xylene and mixed with hydrogen over platinum on silica-alumina not only isomerizes xylenes but also converts ethylbenzene, thus preventing buildup of this compound in the separation-isomerization loop.
The manner of producing p-xylene by a loop including Octafining can be understood by consideration of a typical charge from reforming petroleum naphtha. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling Lbs./U.S. Point .degree. F. Point .degree. F. Gal. ______________________________________ Ethylbenzene -139.0 277.1 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Individual isomer products may be separated from the naturally occuring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes. Of the xylene isomers, i.e., ortho-, meta and para-xylene, meta-xylene is the least desired product with ortho- and para-xylene being the more desired products. Para-xylene is of particular value being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers such as "Dacron".
Isomerization processes operate in conjunction with the product xylene of xylenes separation processes. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
A typical charge to the isomerizing reactor may contain 17 wt. % ethylbenzene, 65 wt. % m-xylene, 11 wt. % p-xylene and 7 wt. % o-xylene when it is desired to co-produce orthoxylene and para-xylene. When para-xylene is desired as the sole product, a typical charge may contain 20 wt. % ethylbenzene, 51 wt. % m-xylene, 9 wt. % para-xylene and 20 wt. % orthoxylene. The thermodynamic equilibrium varies slightly with temperature. The objective in the isomerization reactor is to bring the charge as near to the equilibrium concentration as may be feasible consistent with reaction times which do not give extensive cracking and disproportionation.
In Octafining, ethylbenzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethylbenzene to benzene and diethylbenzene, hydrocracking of ethylbenzene to ethane and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethylbenzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethylbenzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.) has but a very small effect on ethylbenzene approach to equilibrium.
Concurrent loss of ethylbenzene to other molecular weight products relate to % approach to equilibrium. Products formed from ethylbenzene include C.sub.8.sup.+ naphthenes, benzene from cracking benzene and C.sub.10 aromatics from disproportionation, and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than does ethylbenzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, toluene, C.sub.9 aromatics and C.sub.5 and lighter hydrocracking products.
Ethylbenzene has been found responsible for a relatively rapid decline in catalyst activity and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethylbenzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
Because of its behavior in the loop manufacture of p-xylene, or other xylene isomer, ethylbenzene is undesirable in the feed but is tolerated because of the great expense of removal from mixed C.sub.8 aromatics. Streams substantially free of ethylbenzene are available from such processes as transalkylation of aromatics having only methyl substituents. Thus, toluene can be reacted with itself (the specific transalkylation reaction sometimes called "disproportionation") or toluene may be reacted with tri-methyl benzene in known manner.
A recent development in vapor phase isomerization is described in U.S. Pat. No. 3,856,872 (Morrison) dated Dec. 24, 1974. It is there shown that use of a catalyst containing HZSM-5 is very efficient for isomerization of xylenes at reduced hydrogen flow as compared with Octafining. The extent of xylene loss is substantially reduced by this change of catalyst. Concurrently, the mechanism of ethylbenzene conversion is drastically changed on substitution of, e.g., NiHZSM-5, for the platinum on silica/alumina of Octafiners. The Morrison process results in conversion of ethylbenzene by transalkylation reactions including disproportionation of ethylbenzene to benzene and diethylbenzene, disproportionation and ethylation of xylene and the like producing alkyl aromatic compounds of nine or more carbon atoms (C.sub.9.sup.+) together with benzene and toluene. Those conversion products are readily separated in the loop for recovery of p-xylene and isomerization of o- and m-xylenes. In general, loss of xylenes increases as severity of the isomerizer is increased to enhance the conversion of ethylbenzene. Xylene isomerization may also be carried out in the liquid phase, using a similar crystalline aluminosilicate zeolite catalyst, as described in U.S. Pat. No. 3,856,871.